Response of Soft Colloids and Macromolecules to Crowded Environments: Theoretical and Computational Modeling

North Dakota State University Fargo, Fargo ND

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical and computational research and education that is aimed to better understand and facilitate the design of smart materials composed of building blocks whose properties, such as size, shape, and softness, respond and adapt to changes in external stimuli. Promising types of building blocks are soft colloidal particles and flexible molecules containing a large number of atoms called flexible macromolecules, objects whose sizes range up to microns — thousands of times larger than an atom, yet visible only under a microscope. Of particular interest are microgels — microscopic gel particles, made of porous, elastic networks of chain-like molecules, which swell by absorbing solvent. The degree of particle swelling can be controlled by adjusting temperature, acidity, and concentration of the solution, resulting in unique, adaptive properties that equip microgels to serve as biosensors and as vehicles for delivering drugs to the body. When crowded by one another in concentrated solutions, flexible macromolecules can change their size and shape. Interdisciplinary research is driven by fundamental interest in the basic structure of materials and by practical applications in biomedical, pharmaceutical, and foods industries. While past experimental and modeling studies have explored elastic properties of single particles and collective behavior of bulk solutions, an outstanding challenge is to link single-particle or microscopic and collective or macroscopic properties of these materials. Specific unresolved issues concern the role of electric charge in determining swelling of microgels and the influence of particle softness on macromolecular crowding in biological cells. Motivated by the ultimate goal of explaining experimental observations and designing new materials, the research team is developing efficient modeling methods to achieve several major objectives: (1) Elucidate impacts of macromolecule flexibility on materials properties by modeling concentrated mixtures of microgels of different size and softness. (2) Analyze transitions from liquid to solid phases by modeling crowded solutions of deformable microgels and identifying conditions that favor formation of crystalline solids over glassy solids. (3) Explore how different forces between particles determine thermal properties of soft materials by modeling concentrated solutions of microgels with electric charge. A potentially transformative element of the research is incorporation of size and shape fluctuations of macromolecules, which are especially important in overcrowded environments, where soft particles can deform and interpenetrate. These objectives will be achieved by developing and applying an array of modeling methods, including molecular-scale computer simulations, and directly comparing predictions with experimental measurements. Broader scientific, educational, and societal impacts of this project include (1) development of novel computer simulation algorithms and theories, of interdisciplinary value, for modeling dispersions of flexible macromolecules, which can guide the interpretation of experiments and the design of smart, responsive materials, with potential applications to drug delivery, biosensors, and water filtration; (2) science outreach activities with students at K-12 schools and tribal colleges; and (3) mentoring and training of students for careers in computational materials research. TECHNICAL SUMMARY This award supports theoretical and computational research and education that is aimed to bridge our understanding of microscopic properties of flexible macromolecules and emergent macroscopic properties of soft materials. Soft colloidal particles and flexible macromolecules, including microgels and polymer coils, have drawn intense interest as components of smart materials with tunable properties that respond to changes in external stimuli. Particular attention has been focused on microgels, that are natural or synthetic microscopic gel particles, composed of porous, elastic networks of crosslinked polymers, which are swollen by a solvent. When dispersed in water, microgels can acquire charge via dissociation of counterions or from initiators during synthesis reactions. Permeability to solvent molecules and small ions renders equilibrium particle sizes highly sensitive to competition between osmotic, elastic, and electrostatic forces. The degree of particle swelling can be controlled by adjusting temperature, pH, ionic strength, and concentration, resulting in unique, adaptive properties, and making microgels well-suited to drug delivery and chemical and biosensing. In concentrated dispersions, microgels can crowd one another or other macromolecules, or can be crowded by hard nanoparticles, resulting in conformational changes. Interdisciplinary research is driven by fundamental questions concerning self-assembly and practical applications in biomedical, pharmaceutical, and foods industries. Modeling of microgels has been mostly limited to explicit models of single particles or coarse-grained models of many-particle dispersions, while studies of macromolecular crowding have largely neglected shapes of polymers. Linking single-particle microstructure and interparticle forces with macroscopic properties remains an outstanding challenge. Specific unresolved issues concern the role of electrostatics in determining swelling of microgels and the influence of particle softness and deformability on macromolecular crowding, depletion forces, and phase behavior, including stability of crystalline vs. glassy solids in overcrowded dispersions. Motivated by these challenges, and with the ultimate goal of explaining and interpreting experimental observations to facilitate the design of new materials, this project is developing efficient, multi-scale modeling methods to directly address several major objectives: (1) Elucidate impacts of varying internal architecture and compressibility of microgels on swelling in crowded environments by modeling single particles with nonuniform crosslink density and crowded dispersions. (2) Analyze liquid-solid transitions of soft colloids by modeling crowded dispersions of deformable microgels and computing structural properties and phase diagrams to identify system parameters favoring equilibrium stability of crystals over glassy solids. (3) Explain observations of self-healing in overcrowded dispersions of soft colloids and the influence of particle compressibility on bulk structural properties by modeling asymmetric mixtures of microgels. (4) Determine the role of fixed charge density and electrostatic interactions in swelling and thermal behavior of soft colloidal materials by modeling crowded dispersions of ionic microgels. (5) Analyze impacts of steric and electrostatic forces between soft crowders on macromolecular conformations in crowded mixtures of polymer coils and nanoparticles. (6) Explore influences of macromolecular crowding on depletion forces, swelling, structure, and phase behavior in microgel dispersions and mixtures of penetrable microgels and hard nanoparticles. A potentially transformative element of the research is the incorporation of size and shape fluctuations of soft colloidal particles, which are especially important in overcrowded environments, where volume fractions exceed hard-sphere close packing. These objectives are being achieved by developing, validating, and applying an array of computational and theoretical methods, including effective interaction and Poisson-Boltzmann theories and molecular dynamics and Monte Carlo simulations, and directly comparing predictions with experiments. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.